Different behavior when a breakpoint is hit.

Today's SP mystery: I have a workflow that works as expected when run, with or without the debugger attached, unless it hits a (certain?) breakpoint, in which case the UI says:

The workflow failed to start due to an internal error.

Why? I dunno!

EDIT: Upon further testing, I found the workflow to work only intermittently when not in debug mode. Upon revising my build scripts (most notably setting all dependent projects' Copy Local properties to True (even though I always thought that was default behavior), everything worked right.